Interpretive Summary: Deep percolation losses in irrigated agriculture may have a lower salinity than the shallow ground water underlying the area. If this water does not mix with the underlying water it may a resource available for in-situ use by a crop even though the regional ground water is not usable. A study was undertaken to determine whether this phenomena occurs and to what extent. It also has implications for water quality sampling and soil salivation. Intensive sampling of short-term temporal and spatial variations in water table height and salinity under irrigated agriculture revealed significant fluctuations of both parameters in shallow groundwater environments. Changes in the position of the water table corresponded to fluctuations in the salinity both at the top of the water table and at specific depths down the water column. It is therefore critical to take time and depth weighted groundwater salinity measurements under fluctuating water table conditions rather than sampling single piezometers or depth-integrated testwells to assess crop water uptake and salinisation rates. Variations in the salinity of the top of the shallow ground water indicate that the rates of capillary upflow and subsequent soil salinisation determined by lysimeters studies may not be accurately extrapolated to the field scale if limited measurements of the temporal and spatial variations in water table height and salinity are made. Stratification was evident at all 3 sites due to the infiltration of fresher irrigation water overlying more saline groundwater. This stratification, within 1-2m of the ground surface, was apparent at each irrigation event and was found to persist throughout the irrigation season. A combination of variable degrees of mixing between the irrigation water and the saline groundwater as well as vertical changes in the soil matrix is thought to be the primary cause.

Technical Abstract:
Capillary upflow from shallow saline ground water is a significant contributor of soil salinisation in arid irrigated areas and is highly dependent on the water table depth and ground water salinity. Lysimeters have traditionally been used to quantify capillary upflow from shallow water tables and the potential salinisation risk. However, water table depth and salinity generally remain constant in lysimeters, while short-term variations in these parameters exist under surface irrigated agriculture. Consequently, the extrapolation of calculated crop water uptake and potential soil salinisation from lysimeters to field conditions may be misleading. A multilevel sampler was designed to collect depth specific water samples to investigate changes in the height and salinity of a shallow fluctuating water table under furrow irrigated areas. Water samples were taken from the top of the shallow ground water as it rose and fell following irrigation events at three farms in the Murrumbidgee Irrigation Area, in south-western New South Wales, Australia. All sites were furrow irrigated, resulting in large fluctuations in water table height over the irrigation season. The water table was within 2m of the ground surface at all sites. Salinity stratification was found to exist within the shallow ground water with low salinity irrigation water overlying the more saline groundwater with a zone of variable salinity extending to 2.5m. The salinity generally increased with depth. In addition to changes in groundwater salinity with depth, fluctuations in salinity occurred at individual depths over the irrigation season. The salinity of the top of the shallow groundwater also varied with changes in water table depth. Following an irrigation event, the rise in water table height was associated with a decrease in the salinity at the top of the ground water and a water table decline corresponded to an increase in salinity in the upper layers of the ground water. The water table position is an important determinant of salinity risk since this is the foundation for capillary upflow. Consequently, short-term variations in the depth and salinity of shallow water tables under field conditions would affect capillary upflow and salt movement into the root zone. These parameters should therefore be measured at the field scale where possible, to obtain accurate measurements of crop water uptake and potential soil salinisation.